1. Field of the Invention
The present invention generally relates to video recording systems and, more particularly, to systems for providing video special effects during playback of recorded video information.
2. State of the Art
Generally speaking, television pictures are comprised of snapshot-like "frames" that contain video signal information in horizontal scan lines organized by synchronizing signals. For example, according to the standards of the National Television Systems Committee (NTSC), each frame of video information comprises 525 horizontal scan lines. Further according to NTSC standards, the frame repetition rate is thirty frames per second, or 15,750 horizontal scan lines per second.
In practice, all of the video information in a complete television frame is not reproduced at a receiver simultaneously; instead, a technique known as interlaced scanning is used to reduce flicker. In interlaced scanning, each frame of video information is divided into two interlaced fields, each comprising a raster array of odd numbered or even numbered horizontal scan lines. Thus, if the horizontal scan lines of a 525-line NTSC frame were numbered sequentially from the top of a raster array, an odd-line field would comprise numbered horizontal scan lines 1, 3, 5, and so forth through frame line 525. Similarly, an even-line field would include numbered lines 2, 4, and so forth through line 524. According to the NTSC format, the field repetition rate is sixty fields per second.
In video recording systems that employ magnetic videotape as the recording medium, it is conventional to record each television field by using only a single track on the recording medium as traced by a recording head of the helical type. (Helical recording heads are widely used because they provide high head-to-tape speed, usually exceeding one meter per second, with relatively slow moving tape.) Thus, FIG. 1 shows an example of four fields of video information recorded in parallel tracks 2, 4, 6 and 8, respectively, which extend at a small angle relative to the longitudinal edge of a magnetic videotape 10.
It is well known, of course, that videotape recorders can provide special effect capabilities. One common special effect is a "still" feature that "freezes" a recorded television frame for viewing during playback. Other common special effects include "slow" and "fast" scan features. A slow scan feature, which enables a viewer to scan through recorded video information at speeds slower than normal playback speeds, is obtained by reducing the relative speed between a playback head and magnetic videotape.
A fast scan feature, which enables a viewer to scan through recorded video information at speeds faster than normal playback speeds, is obtained by increasing the relative speed between a playback head and a magnetic videotape. For instance, FIG. 1 illustrates the case of a fast scan of a magnetic videotape by a helical playback head that follows an accelerated path 12. In practice, fast scans often distort moving images and changing scenes because, as shown in FIG. 1, the playback head picks up information from different tracks (i.e., fields) during the fast scan.